As network communications grow more common, it becomes increasingly important to use network bandwidth in the most efficient way possible. In general, data communication through multiple networks is accomplished in one of two ways: routing each portion of the data through the network, called a routing system, or establishing a complete path through the network, called a connection. Each of these systems has its own advantages and disadvantages.
Routing System Transport
A routing system typically consists of tables of data that describe which computers are directly accessible to all other computers. The routing system examines a header within the packet to determine the destination address of a packet and which node in the network to transmit the packet to next, i.e. the destination of the next hop in the path from the source to the destination nodes. Ultimately, the data packet reaches the destination by being examined by each node in the path to the destination. The header within the packet contains an identifier that can be used to determine some of the characteristics of the packet. The resulting overhead in determining the paths on the routing system can reduce performance of data transport from the source to the destination. Consequently, a routing system works well if the data to be transported is relatively small or is concentrated at one point in time. One example of a routing system is transport control protocol/internet protocol ("TCP/IP" ). TCP/IP does not utilize any specific predetermined path from a message source to a message destination. In TCP/IP, messages are routed based upon a destination address contained in the message. The message may take any path to the destination which is available. The message may also contain the address of the source of the message but typically does not contain the address of the devices in the path from the source to the destination. These addresses are determined by each device in the path based upon the destination address and the paths available.
FIG. 1 illustrates a typical interconnection of networks. As seen in FIG. 1, a source node 10 is connected to a destination node 35 by a number of networks, 12, 13, 14, 21, 22, and 31 and routing devices 15, 20, 25, and 30. When operating using TCP/IP, source node 10 sends a TCP/IP datagram to the destination node 35 by forwarding the datagram through a series of routing devices. Each subsequent routing device has information regarding networks and other routing devices that may be used to forward datagrams. The routing device 15 forwards the datagram based upon a destination address contained in the datagram by looking up in a routing table the address of the device where the datagram is to be sent. This routing table associates destination addresses with the addresses of accessible devices. Thus, in the present example, if the datagram from node 10 has a destination address specifying node 35, then routing device 15 may direct the datagram across network 13 to routing device 25, which in turn would route the datagram over network 22 to node 30, which would finally route the datagram to the destination node 35 over network 31. Each step in the path of the datagram from node 10 is referred to as a "hop". A routing table specifies the address of the next hop in a datagram's path based upon the destination address of the datagram. Thus, the routing table includes the address of accessible devices to the routing device and the destination addresses which are associated with those accessible devices.
The manner in which a routing table is utilized by a routing device is known as a "routing algorithm". In TCP/IP, the routing algorithm is typically a three step process where the first step is to determine from the destination IP address of the datagram whether the address appears among the direct routes (host devices or networks attach directly to the routing device) specified in the routing table. If such is the case, then the datagram is sent to the directly attached network. If the destination address does not specify a direct route, then the routing device determines if an indirect route (a route through a gateway to a device or network not directly connected to the routing device) is specified for the destination address. If such is the case, then the datagram is forwarded to the specified gateway IP address. Finally, if no direct or indirect route is specified, then the datagram may be sent to a default address or, if no default address is specified, an error message is returned to the source. The routing tables used by routing devices in TCP/IP networks may be arrived at in a number of ways. In one type of routing, a routing table is broadcast to each routing device from the device which maintains a routing table of all possible routes. In other forms of routing, each routing device builds a routing table via its own dialog which is called the routing protocol. These tables are then built and maintained by each routing device. In a third form of routing, a static routing table is used for each message routed.
Each of these cases may have its own disadvantages. For example, the broadcast and routing protocols are repeated on a routine basis to assure the routing device has an accurate routing table. Consequently, this broadcast may occupy network bandwidth. The static routing table, while not requiring network bandwidth, may be inefficient in determining the optimal routing path or, in a worse case, be unable to adapt to changes in the network topography which makes routes unavailable.
With respect to the dynamic routing methods, as the number of interconnected networks and number of devices connected to those networks increases, the size of the routing table and therefore the amount of network resources occupied to maintain the routing table may also increase. In certain circumstances, this overhead may occupy so much of the resources of the network as to cause unacceptable reductions in network performance. Thus, in a routing system, communications are individually routed which incurs overhead of routing each datagram at each network node. Routing may, therefore, be less than optimal for long communications between the same sources and destination nodes. There are, however, certain advantages to a routing system. Specifically, a routing system may have the advantages of reliability and bandwidth utilization. For example, a routing system may achieve increased reliability by its ability to route a message from a source node to a destination node along different or alternative paths. Furthermore, a routing system may achieve greater bandwidth utilization by using only that bandwidth which is necessary to transport the messages required.
Connection Oriented Transport
A connection oriented system consists of virtual connections that link source nodes to destination nodes. The virtual connections are configured early and exist until the data transmission is complete. Data is transported to a destination node by examining the header that indicates which virtual connection to use. Unlike a routing system, nodes in a connection oriented transport do not need to examine the packet to determine which node to route the data to next. Rather the complete connection is established in advance and maintained until the data transmission is complete. The connection oriented transport, however, allocates a discrete portion of the network's bandwidth for each virtual connection. If a virtual connection is not used properly, bandwidth may be wasted. A connection oriented transport works well if a large amount of data is to be transported.
One example of a connection oriented transport is asynchronous transport mode (ATM). ATM transports data by establishing virtual connections between the source and destination nodes. Unlike the routing systems described above, data transported by ATM has a channel allocated to its use. In other words, the datagram in a routing system only specifies its source and destination while a connection oriented system specifies the route the data follows across the network to reach the destination. Such a system is illustrated in FIG. 1. Source node 10 requests the connection to destination node 35 from network device 15. The connection request specifies several connection parameters, including destination, traffic type, bandwidth requirement, delay and cell loss requirement. Network device 15 determines which source to destination route best fits the parameters specified. Network device 15 then selects a virtual connection to the next network device and sends a connection setup request to that node. The network device receiving the request then selects its own virtual connection to the next network device and associates the virtual connection from network device 15 with the virtual connection it selected to the next network device. This process continues until the destination is reached and a complete virtual connection is established between the source and destination nodes. The destination node responds by returning an acknowledge (ACK) signal to the network device making the request. Each network device then returns the ACK to the network device that made a request to it. The ACK is ultimately transmitted back to the source node completing the connection setup.
For example, assume that network device 15 determines that the best connection to destination node 35 is across networks 13, 22, and 31. Network device 15 sends a connection request to network device 25 across network 13 using virtual connection 1. Network device 25 then selects virtual connection 2 across network 22 to network device 30. Network device 25 then associates the virtual connection between network devices 15 and 25 with the virtual connection between network device 25 and 30. In other words, network device 25 associates virtual connection 1 with virtual connection 2. Network device 25 then sends a connection setup request to network device 30. Network device 30 then selects virtual connection 3 to the destination node and associates it with the virtual connection 2. This completes the virtual connection in the forward direction.
The destination node then responds by sending an ACK to network device 30. Network device 30 sends an ACK with an identifier for virtual connection 3. Network device 25 then associates virtual connection 3 with virtual connection 2 going in the reverse direction. Network device 25 sends an ACK to network device 15 along with the identifier for virtual connection 2. Network device 15 then associates virtual connection 2 with virtual connection 1 going in the reverse direction. This completes the virtual connection from the destination to the source node. These connections in the established path are then utilized for data transmission without further reference to the individual packets. Thus in a connection oriented transport system communications are routed from the source node to the destination node without the overhead associated with a routing system. This has the advantage of relieving the individual nodes along the path specified from the source node to the destination node of the overhead of having to route each portion of the datagram. Thus the individual nodes along a routing path are relieved from the task of having to route messages from node to node.
There are, however, disadvantages associated with a connection oriented transport. Specifically, a complete connection using a connection oriented transport has a discrete amount of network bandwidth associated with it. Consequently, if a particular connection is not fully utilized, some network bandwidth is wasted. Furthermore, the overhead associated with establishing the virtual connection from the source node to the destination node may also cause network bandwidth to be wasted.
The best transport for the data should be chosen to make the most efficient use of the network resources. It is possible, however, for the nature of the data to change after the selection has been made. Consequently, the best transport choice can also change. For example, if a routing system was chosen originally and the nature of the data changes such that a connection transport would be more efficient, network resources would be wasted despite having made the proper choice at the outset. Consequently, it is desirable to switch from one transport mechanism to another (i.e., switch from a routing system to a connection oriented system if the switch would result in better use of network resources). One way to switch from one transport mechanism to another is to count the data packets transmitted and switch when the count exceeds some predetermined number. This method, however, is simplistic and does not directly address the problem in terms of the cost of network resources and may result in inefficient use of network resources. Given the important nature of network resources and the differences in performance between routing and connection oriented systems, there is a need for an improvement in how the most efficient type of transport is selected.